The present invention relates to a method for the manufacture of slabs of ceramic material and, more specifically, to an improvement relating to the method and plant described, illustrated and claimed in Italian patent No. 1,293,176 filed on Apr. 15, 1997(corresponding to WOA-9846453) of the same applicant.
The slabs of ceramic material to which both the method according to the present invention and that of the above mentioned patent refer are the subject of U.S. Pat. No. 5,264,168and manufactured from a ceramic mixture consisting of a granulate complying with specific parameters as regards particle size and of a binding phase which a special composition characteristics and for the details of which reference should be made to the text of the aforementioned U.S. patent.
During the implementation of said method it was noted that some stages posed certain problems as regards an industrial production, said problems having been solved with the method and plant according to the above mentioned Italian patent.
This latter method and plant envisaged the following operations:
1. Depositing a layer of fabric (felt) onto the moulding support;
2. Arranging a sheet of a paper permeable to water vapour on top of the fabric layer,
3. Deposing the ceramic mixture onto the sheet of paper, if necessary in two separate stages with insertion, after the first stage, of a mesh of reinforcing material to be embedded in the slab body;
4. Depositing a sheet of rubber onto the layer of mixture;
5. Vibrating compression under vacuum performed above the rubber sheet;
6. Removal of the rubber sheet;
7. Transfer of the xe2x80x9cvirginxe2x80x9d slab onto a metal-grid support by means of pincer means gripping the edge of the felt;
8. Drying treatment of the slab;
9. Raising of the dried slab and removal of the fabric layer,
10. Applying a layer of refractory material (engobe) onto the upper surface of the dried slab (previously lined with the rubber sheet) and drying thereof,
11. Overturning the slab so that it rests on the engobe-lined surface and introduction into the firing kiln, with simultaneous burning of the paper sheet still adhering to the now visible surface of the dried slab.
The slab thus obtained then undergoes the usual finishing operations, such as sizing, polishing, etc.
A major problem during this production process has been that of performing drying of the slab after the moulding phase (namely after vibrating compression under vacuum) in a rapid and as homogeneous as possible manner.
It is obvious that, in order to achieve this object, the surface of the moulded slab must be as free as possible and therefore exposed to the action of the drying means (for example hot air). However, the xe2x80x9cvirginxe2x80x9d slab is necessarily formed on a support capable of withstanding a vibrating compression and cannot be harped before, because of drying it reaches a sufficient degree of rigidity so as to become self-supporting at least for performing the albeit minor handling operations required in order to convey it to the final firing stage.
For this reason, the technology applied hitherto and described in the aforementioned Italian patent uses a layer of fabric or felt which allows the passage of the water vapour from the mixture and a sheet of paper arranged between felt and mixture, said sheet performing multiple functions, namely:
(i) physically separating the mixture from the felt or fabric;
(ii) absorbing excess water, which is mainly naturally expelled from the mixture layer during the vibrating compression stage, and
(iii) preventing the formation of folds which are also due to the mixture water and which could red in bending of the final slab.
For this reason, the method according to the above mentioned Italian patent uses a thin sheet of paper permeable to water vapour which is arranged above a layer of fabric, preferably felt, which also performs the function of absorbing and allowing the water to pass through during drying.
In the practical implementation of this method, the thin sheet of paper, which is completely saturated with water, is preferably treated so that, after drying, it does not form folds which could damage the final slab.
It has now been found, and accordingly is the subject of the present invention, that, by modifying certain stages of the above mentioned method and altering the nature of one of the elements used in the said method, the latter is substantially improved.
These modifications consist mainly in replacing the thin sheet of treated paper with a sheet of paper of considerable thickness, which is usually classified as cardboard or paperboard, depending on the thickness and use, so as to absorb all the excess water without forming folds after vacuum vibro-compression and the drying stage. Herebelow, for the sake of simplicity, this sheet will be simply referred to as xe2x80x9cpaperboardxe2x80x9d.